In a large pot heat butter with leeks and garlic, sweat on medium low heat until translucent but not browned, about 5 minutes. Add potatoes, broth and bouquet garni and simmer, uncovered for 17- 20 minutes or until potato is very tender. Remove bouquet garni and discard.
In a blender, purée soup or with immersion blender, until very smooth (add more stock if too thick). Add the cream, season with salt and white pepper. Bring back to a simmer over low heat. Serve with spoon of crème fraiche and fresh parsley.